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high speed laser diode

jayanthyk192

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Hi,

I'm in need of a high speed laser(any color) for digital switching.Speed required is around 60Mhz.Power of about 50mW or greater.Please give me links where i can find the laser diode with lens assembly.

Thank you.
 

Sigurthr

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What type of modulation do you need? I don't think laser diodes are fast enough to reach 60MHz TTL, but analog AM modulation sure, that shouldn't be a problem.

Thing is... if you're able to design a driver that can accurately modulate the current with an AM signal at 60 megacycles...you normally wouldn't be asking on a hobbyist forum about sourcing a diode. This leads me to believe you might be asking about TTL, which is problematic with non RF rated (non-light/laser)diodes. The junction capacitance in most diode types is just too high to go from off to on that quickly; above a certain frequency you start missing cycles. 60MHz is pretty high up there, that's the 5 meter band.

What is your intended application?
 

jayanthyk192

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@Sigurthr

Yes,I want to build the ttl circuit.But I'm guessing its quite difficult to find a laser with such high switching speeds.So,I' thinking of going with the AM as you suggested.What are the maximum operating frequencies of AM that are supported by laser diodes?
 

Sigurthr

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I'm not sure actually, since the junction stays on, and it is just a variance in how much current is flowing, there may not be a real hard limit on frequency. I would think that eventually the stray/inherent inductance would slow and smooth out current changes once you get to a high enough frequency though. I don't know for sure, but I suspect you could get AM modulation up to the Ghz range with properly designed driver technology. That would take some complex / expensive stuff though.

I've built home made DIY AM laser transmitters which can handle up to 1mHz modulation frequency on paper. In practice I've never tried above 35kHz.

Again, as far as I know inductance is the only limiting factor on AM modulation of a constantly forward biased diode current.
 

jayanthyk192

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@Sigurthr
is it same for all laser diodes and for all power ranges?Can i be using a DAC to control the modulation or should be a really smooth sine wave?
 

Sigurthr

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A square or sine wave would be ideal. A square wave would let you get a bit higher frequency than a sine wave as the inductance would smooth out the square wave to that of a sine wave at the same frequency where an input of sinewave would get distorted.

I'm not sure if all diodes are the same, here in the hobbyist realm we don't really get datasheets or even aknowledgement from the laser diode companies, they don't value us. You would have to look at the capacitance and inductance values for the diode and determine the impedance at the desired frequency. Beyond that I do not know, I'm not a PhD in optical communications. I do know IR diodes can be AM modulated very, very quickly with accuracy as this is how high speed fiber optic data is transmitted.
 
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jayanthyk192

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@sigthur
thank you for all the help.I'll run a few tests using a signal generator and find the limits of operation on the diodes i have.
 

Sigurthr

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In another thread Steve of LSRFAQ posted some relevant info:

LSRFAQ said:
If your dead set on green, you can use a Acousto Optic Modulator for the green. You'll have a 7 to 10 Mhz bandwidth in the modulator, and with work, you can also have gray scale.

A normal small 50 to 100 mW red diode is good to almost a gigahertz. Around a Gigahertz, some wired problems with carrier lifetime in the diode start to occur.

Steve
I forgot about AOMs, hehe. I don't think AOMs can give full deflection so to speak so TTL is still out of the picture, but you can get real nice AM modulation with one. And as I had suspected you can get right up to nearly 1GHz with a red diode.

Glad to be of some help. Cheers!
 
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jayanthyk192

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@Sigurthr

This is the first time i'm hearing about AOMs.They seem pretty impressive as per what wikipedia says.Any ideas where i can get these?Hope they're not too expensive.Is q-switching same as AM or like TTL modulation?
 

Sigurthr

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I've never worked with or sourced one, they're before my time mostly, hehe. Generally they are used when direct modulation of the power source will not result in modulation of the laser output, such as with gas lasers or DPSS systems.

Q-Switching is a bit different, it isn't a means of modulation, even though it does produce pulses, the pulses are not controlled for data transmission, more precisely the pulses are what causes the lasing, not the other way around.
 




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